1. Field of the Invention
The invention relates generally to semiconductor fabrication and more specifically to measurement of films and plasma emissions during wafer processing.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to measure material features on substrates. Typically, integrated circuit devices are manufactured in the form of multi-level structures. At the substrate level, transistor devices having p-type and n-type doped regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Dielectric materials, such as silicon dioxide, insulate patterned conductive features. Etching paths through these layers provides a means for interconnecting contacting semiconductor devices such as transistors. Metallization line patterns are formed in dielectric materials, and then metal CMP operations are performed to remove excess metallization.
During integrated circuit fabrication there are many opportunities for gathering metrology data, that is measuring material and device properties on substrates. Many properties can be determined by capturing a signal indicating the device, feature or material. As features and the thickness of films employed in the manufacture of semiconductors continue to decrease in size, the task of collecting metrology becomes more sophisticated and precise. Properties of materials on the substrate are carefully monitored throughout the fabrication process, but the task is more difficult during interlayer dielectric (ILD) stages, that is, when stacks consist of multiple dielectric and metal film layers.
This disclosure relates to the measurement of thin films through the use of standard optical methods utilizing reflections off and transmission through materials and characterization of plasma or chemical emissions during material processing. Optical emission spectroscopy (OES), interferometry, spectral-reflectomety, ellipsometry and other suitable methods employ the use of and characterization of wavelengths of light and have been used extensively in the semiconductor arts. Optical sensors maybe used for non-contact thickness measurement of transparent films, such as silicon dioxide and other materials used in the manufacture of semiconductor devices in addition to classification of plasma emissions (optical emission spectroscopy) during plasma etch operations. In some operations, such as plasma etching, emissions during a process provide real-time monitoring and process control. Optical techniques such as ellipsometry and reflectometry have been used extensively in the semiconductor arts for measurement of thin films (U.S. Pat. No. 4,899,055 “Thin Film Thickness Measuring Method”). Lam Research has been a leader in providing in-situ classification and measurement of plasma etch processes (U.S. Pat. No. 6,160,621 “Method and Apparatus for In-Situ Monitoring of Plasma Etch and Deposition Processes Using a Pulsed Broadband Light Source”).
Transmissive films include a broad range of dielectric and semi-conductive materials that allow certain wavelengths of light to pass through based on the index of refraction and extinction coefficient of the particular material. Illumination by a light source such as a xenon lamp provides wavelengths of light from ultraviolet to near infrared ranges. The selection of the light source may be dependent on the type of films to be measured by the optical sensor. The light source may be pulsed or flashed at defined periods to enable error subtraction (smoothing or averaging) in addition to cancellation of movement induced by the rotation of the substrate and scanning of a sensor. A computer in concert with a spectrograph (described below) can control operation of the strobe, including such parameters as the period of the flashing illumination.
A receiving element, such as a fiberoptic cable 20 shown in FIG. 1, is capable of collecting light returning from the surface of the substrate. The fiberoptic cable 20, may include a core 26 as well as a number of transmission lines 26 all capable of transmitting the light signal. The fiberoptic cable 20 passes the received light to a spectrograph for analysis. The spectrograph may be incorporated in the computer, or may be a standalone unit that serves as input into the computer. A spectrograph includes an arrangement of semiconductors elements that transform light energy into electrical energy such as a charge-coupled device array (CCD) or photo diodes and a method of extracting this position-dependent information.
When used to measure film thickness, the spectrograph utilizes interference of reflected light from a pair of surfaces to determination of the thickness of defined materials. Spectral reflectometry can be used to measure the difference in the optical path length between interfaces, to provide a measurement of the thickness of film layers.
Another technique utilizing linearly polarized light from an illumination source may be used to measure layer thickness. Linearly polarized light (transmitted via a fiberoptic cable) reflected off a thin film, becomes elliptically polarized. Analysis of this change across the spectrum (provided by a spectrometer described below) provides material properties of the film or stack of films such as the thickness and refractive index of the material.
Measurement systems utilizing spectral information, especially those that provide full surface mapping require a tremendous amount of processing power for complete collection, storage and analyzing of data retrieved by the sensors. The problem with known approaches is that the amount of data is necessarily limited by the speed of the associated computer or processor. As processor speeds continue to increase it will be possible to take more samples and reduce the integration time.
In optical emission spectroscopy (OES) plasma diagnostics, spectrographs may be employed in order to monitor and classify emission characteristics during a process such as etching layers of semiconductor material. During etch operations, known reactive gasses are excited and accelerated to the surface of a wafer by high frequency (RF) energy. At the wafer surface the chemicals added to the chamber react and in some cases recombine with material on the surface of the wafer. By-products (incl. recombinants) of the reaction are noticeable in the plasma discharge within the chamber before being pumped away by a vacuum system. A fiberoptic cable 20 positioned to capture the glow of the reaction transmits the signal back to the spectrograph for analysis. The plasma emissions associated with particular reactions (a.k.a. signatures) can be classified in a manner so that known good processes and known failures can later be classified during or immediately after processing operations. Precise plasma monitoring activities require relatively high sample rates and efficient processing which is taxing for many of today's commercially available systems.
The present invention provides a solution to the data collection bottleneck experienced while monitoring, analyzing, and classifying data obtained from in-situ and post-process film measurement, as well as in-situ plasma diagnostics during etch operations.